new-source-dir
An advisory: it explains, and blocks nothing, on a new file that opens a directory, which is a boundary rather than a file.
What it catches
A new file that opens a directory, which is a boundary rather than a file.
Seeing it
A verdict names its rule in brackets, which is how you got here:
advise [new-source-dir]: ...
magus describe rule new-source-dir prints the same entry at a terminal, and
magus describe rules lists every rule this workspace enforces.